Common wire ropes and cables normally feature a metallic core surrounded by an outer layer of helically laid steel wire or wire strands. The cable with metallic core has a disadvantage of being exceedingly heavy in long lengths.
Therefore, ropes with a fiber core of natural or synthetic fibers twisted together with metallic wire strands, i.e. so called hybrid ropes, are introduced to impart various characteristics to the ropes depending on the type of natural or synthetic fibers used.
An advantage of a hybrid rope in view of a fully steel rope is the lower weight of the rope and improved performance like e.g. tension and bending fatigue.
The advantage of the hybrid rope in view of a fully fiber rope, e.g. nylon or polyester is that the hybrid rope is highly resistant to abrasion, crushing and stretch while also exhibiting the desired characteristics of toughness and excellent impact strength.
U.S. Pat. No. 4,034,547-A discloses a composite cable 10 which comprise a synthetic core 12 and a metal jacket 14 as illustrated in FIG. 1. The synthetic core 12 is formed of a bundle of low stretch fibers and the jacket 14 is formed of a plurality of wires or wire strands 16. This patent further discloses that a weight approximate 30% lighter than the weight of the corresponding size steel cable can be achieved by the composite cable.
The advantage of hybrid ropes comes into effect in particular in the case of ropes of great length for suspended use, such as hauling or hoisting operations, ropes in mining, cranes and elevators, aerial ropes or ropes for installations or use in marine and commercial fishing applications, and offshore applications like mooring, installation etc. This is because, during such use, the weight of rope by itself already takes up a large part of its load-bearing capacity and winch load capacity; the payload is correspondingly limited. Therefore, hybrid ropes are desirable in these operations since they provide comparable performance with steel ropes and lower weight expanding the possibilities, e.g. mooring deeper in the water.
On the other hand, however, hybrid ropes having nylon or polyester core do not have high breaking loads, therefore cannot be used where high strength as in case of full steel ropes is required. In such case hybrids with high modulus fibers as core can be used.
It however has the drawback of requiring important modifications relative to more conventional cables as to its use and control. For example, the fiber core is relatively easy to be abraded due to its movement relative to the steel outer layer when the rope is in use. Very recently, international patent application WO-2011/154415-A1 discloses using the coating of plastomer on the high modulus polyethylene (HMPE) core to protect the HMPE core against abrasion due to the movement of the steel wire strands. Moreover, less slippage occurs between the core and the steel outer layer.
However, for critical applications, where huge compressive stresses are created in the rope either from high applied loads, crushing in a winch or a drum winder or when very low bending radius is applied, for instance in conditions D/d≤30 (where D represents the diameter of pulley and d is the diameter of the rope) and SF≤5 (SF is an abbreviation of safety factor), it is found the extruded plastomer is not sufficient to protect the core and the plastomer may deteriorate and will be pressed out between the steel wire strands to the outer rope surface after being used for certain time.